Widely used conventional magnetic recording media such as video tapes, audio tapes, and magnetic disks comprise a nonmagnetic support having thereon a magnetic layer comprising particles of ferromagnetic iron oxide, Co-modified ferromagnetic iron oxide, CrO.sub.2, ferromagnetic metal, or hexagonal ferrite dispersed in a binder. Of these ferromagnetic materials, hexagonal ferrite is known as a material having excellent suitability for high-density recording. The following are examples of magnetic recording media employing magnetic particles of hexagonal ferrite.
JP-A-60-157719, for example, discloses a magnetic recording medium having a magnetic layer which contains magnetic particles having a particle diameter of from 0.1 to 0.3 .mu.m and has a vertical-direction squareness ratio of 0.7 or more and a surface roughness of 0.05 .mu.m or less. (The term "JP-A" as used herein means an "unexamined published Japanese patent application.") This prior art technique is intended to provide a magnetic recording medium for high-density recording which has a sufficiently high vertical-direction squareness ratio and excellent surface smoothness.
JP-A-62-109226 discloses a magnetic recording medium comprising a support having thereon a magnetic layer which has a thickness of 1.8 .mu.m or less, contains platy magnetic particles having an average particle diameter of 0.2 .mu.m or less and an average aspect ratio of flatness of 6 or more, and has a specific vertical-direction squareness ratio and a specific vertical-direction coercive force. This prior art technique is intended to provide a magnetic recording medium which has excellent running durability during use and satisfactory suitability for overwriting and attains high recording density and high output.
JP-A-64-89022 discloses a magnetic recording medium which employs a binder having a saturation magnetization of 60 emu/g or more, a specific surface area by BET method of from 25 to 70 m.sup.2 /g, an average particle diameter of from 0.01 to 0.2 .mu.m, and a coercive force of from 400 to 2,000 Oe, and containing a polar group in an amount of 1.times.10.sup.-5 eq/g or more. This prior art technique is intended to improve reproduced output and attain a high C/N ratio and improved running durability.
JP-A-3-280215 discloses a magnetic recording medium in which the longitudinal-direction coercive force is from 1,000 to 4,000 Oe and the residual magnetization in the longitudinal direction is higher than that in the vertical direction, which in turn is higher than that in the in-plane width direction. This prior art technique is intended to provide a magnetic recording medium for high-density recording which has a satisfactory balance between long-wavelength output and short-wavelength output.
JP-A-5-40370 discloses a magnetic recording medium in which 100 parts by weight of magnetic particles having a specific surface area of from 23 to 45 m.sup.2 /g and a coercive force of from 400 to 2,000 Oe are dispersed in from 10 to 40 parts by weight of a resin binder. This prior art technique is intended to provide a magnetic recording medium for high-density recording which is reduced in noise and has excellent orientation.
JP-A-5-12650 discloses a magnetic recording medium which comprises a support, a magnetic layer containing hexagonal ferrite and having a thickness of from 0.1 to 0.6 .mu.m, and a nonmagnetic layer provided between the magnetic layer and the support and having a larger thickness than the magnetic layer. This prior art technique is intended to improve surface properties, short-wavelength output, erasion characteristics, and durability.
JP-A-5-225547 discloses a magnetic recording medium comprising a nonmagnetic support, a nonmagnetic layer provided thereon, and a magnetic layer provided on the nonmagnetic layer and containing 0.1 .mu.m or less magnetic particles. This prior art technique is intended to provide a magnetic recording medium having excellent high-frequency electromagnetic characteristics, satisfactory suitability for signal overwriting, and good durability.
In JP-A-3-286420, IEEE. Trans. Mag., Vol. 24, No. 6, Nov. 1988, p. 2850, there is a description to the effect that the electromagnetic characteristics of a magnetic recording medium containing hexagonal ferrite are influenced by the anisotropic magnetic field HK of the ferrite. The former reference discloses a magnetic recording medium which has two magnetic layers provided on a nonmagnetic layer and in which the lower magnetic layer has an axis of easy magnetization in the longitudinal direction and the upper magnetic layer contains magnetic particles having an anisotropic magnetic field of 3,000 Oe or less; this prior art technique is intended to provide a magnetic recording medium which attains high output over a wide range from a long-wavelength region to a short-wavelength region.
Furthermore, a large number of inventions concerning a squareness ratio SQ in magnetic recording media are disclosed in JP-A-60-164925 and JP-A-3-49025.
However, the invented prior art recording media employing hexagonal ferrite described above have failed to fully exhibit their performances although effective in some degree. There has been much room for an improvement in output, especially in the ultrashort-wavelength region (usually, recording wavelengths of 0.5 .mu.m and less), but conditions under which such improvement is attained have been unable to be found.